Avoid "operation may be undefined" warning in remote.c

GCC 4.1 gives the following warning:
gdb/remote.c: In function 'remote_parse_stop_reply':
gdb/remote.c:6549: warning: operation on 'p' may be undefined
on this line of code:

	event->ptid = read_ptid (++p, &p);

Since p actually isn't used afterwards anyway, simply use NULL.

gdb/
	* remote.c (remote_parse_stop_reply): Avoid GCC 4.1 "operation
	may be undefined" warning.